This invention relates to semiconductor diode lasers, and more particularly to a lead-salt diode infrared laser having a substantially increased frequency tuning range. It also involves an improved method of making such a laser.
A semiconductor diode laser is usually formed in a monocrystalline semiconductor body having a PN junction and two mutually parallel reflective faces that are perpendicular to the PN junction. The semiconductor body is usually a rectangular parallelepiped, and the reflective surfaces form a laser cavity adjacent one side of the PN junction. However, the cavity does not have to be formed in a rectangular parallelepiped body, or even in a body with flat parallel faces. Ring-type lasers, cylindrical lasers, and others are known.
The lasing action is produced by applying a forward bias voltage across the PN junction. The forward bias injects electrons across the PN junction to stimulate emission of radiation. Above a given level of electron injection, called threshold current (J.sub.TH), emitted radiation is collected and amplified in the laser cavity. The amplified radiation exits the laser cavity parallel the PN junction as a monochromatic and coherent beam. The radiation wavelength emitted by a laser is essentially a function of the semiconductor band gap. Composition of the semiconductor material primarily determines the band gap. However, composition is not the only factor which affects band gap. Laser body operating temperature, injection current, magnetic fields and pressures also affect the band gap. They can be used to precisely adjust the principal active radiation mode of a laser to a preselected wavelength. Such adjustment is referred to herein as tuning.
Semiconductor diodes will function as lasers at very low temperatures. For example, lead-salt lasers commence lasing action at only a few degrees Kelvin. As laser temperature increases, so does the frequency of the radiation which is emitted. On the other hand, electrical and/or radiation losses also increase with increasing temperature. As a result the threshold current (J.sub.TH) also increases with increasing temperature. The threshold current, as mentioned above, is the current level at which lasing action commences. Such losses in efficiency not only reduce laser output power but also require higher input power to initiate lasing action. At some point the progressively increasing losses become so large that lasing action will not even start. Accordingly, attempts have been made to reduce some of these losses, as for example by growing higher quality crystals, improving crystal processing to maintain high quality, providing lower resistance ohmic contacts, providing better laser cavities, and better heat sinking of the laser body.
In my jointly authored paper Lo et al, "IngotNucleated Pb.sub.1-x Sn.sub.x Te Diode Lasers", J. of App. Physics, v 47, n 1, pp 267-271 (January 1976) a method of making higher quality lead-salt crystals for lasers is described. In a later paper, Lo, "Tellurium-Rich Growth and Laser Fabrication of Lead-Tin-Telluride", J. of Electronic Materials, v 6, n 1, pp 39-48 (January 1977), I describe growing high conductivity P-type lead-salt crystals and making improved lasers with such crystals. Lead-tin-telluride lasers were made using the cadmium diffusion process described in my paper "Cd-Diffused Pb.sub.1-x Sn.sub.x Te Lasers with High Output Power", App. Phys. Letters, v 28, n 3, pp 154-156 (February 1976) and covered by my U.S. Pat. No. 4,064,621 Lo.
I have now found how to improve semiconductor diode lasers even further, especially lead-salt diode lasers. I have found that if the lead-salt laser cavity has an increasing majority carrier concentration in a direction extending away from the PN junction, lower threshold voltages and increased frequency tuning range can be obtained. I have found that a graded majority current carrier concentration can improve homojunction lead-tin-telluride diode lasers to provide threshold voltages that are significantly less than heterojunction lead-salt diode lasers. I believe that a principal effect of the graded carrier concentration is to provide improved optical confinement with the laser cavity. I also believe that improved optical confinement is attributable to an increasing index of refraction in the semiconductor crystal in a direction extending away from the PN junction of the laser cavity. Such improved lasers and a method for making them are hereinafter more fully described. They are also described in my paper, Lo, "Homojunction Lead-TinTelluride Diode Lasers with Increased Frequency Tuning Range", IEEE J. of Quantum Electronics, v QE-13, n 8, pp 591-595 (August 1977).